Yamaha G-100 secondary fuse

[KM1]

Hey all!! I was given a Yamaha G-100 amp that I am desperate to get going. Here's the skinny: Previous owner was carrying out after a shore over his shoulder and was getting zapped by the power cord when it would make contact with his back. He retired it to a dusty corner of his studio. Someone told him it sounded like the capacitors. That's all the info I got. I checked the capacitors and they are good (thank god, because it is a bitch trying to find a single 1000uf/160v capacitor). There are two bus fuses in the back, a primary and a secondary. The secondary blows as soon as you power it on. When it comes to electronics I am at the kindergarten level but can figure it out if I knew where and what to look at. Surely, someone here must have an idea what my problem is. Thank you for your help!!!


KM1

[52 Bill]

Welcome to the place.

Blown fuses means that there is something drawing too much current in the amp. Typical causes are shorted output transistors and shorted power supply diodes.

Search for a schematic that is correct for your amp and post it or a link so that we all can know what you have there. It will also make it easier to discuss specific part numbers, etc.

[KM1]

Welcome to the place.

Blown fuses means that there is something drawing too much current in the amp. Typical causes are shorted output transistors and shorted power supply diodes.

Search for a schematic that is correct for your amp and post it or a link so that we all can know what you have there. It will also make it easier to discuss specific part numbers, etc.

here is the manual. mine is the 410 US model.

[52 Bill]

So the 3 amp secondary fuse has blown, which only feeds the power amp module. That would suggest that there is a shorted output transistor.

The amp will power up without the secondary fuse, so if you have the means to safely measure the dc voltages, turn on the amp and read the voltages that are on the three power supplies. They are all positive and are +12vdc, +36vdc and B+, which is not listed. My guess would be somewhere around +60vdc to +80vdc.

For the next step you will need to pull out the power amp module and get to the output transistors to test them. As I recall there are two large screws that bolt down the heatsink to the chassis that need to be removed. Once they are removed, the module pulls straight out from the plug.

The power transistors are mounted on the heatsink and plug into sockets on the pc board. The fast test is to diode test them at the three solder terminals that are on the pc board. If either one tests as shorted, then remove it and retest out of circuit.

Let us know what you find out.

[KM1]

Thank you so much 52 Bill!!! I will let you know how it goes. Thank you!!!

[52 Bill]

I was reading through the manual that you have posted and noticed that there are very good instructions about how to remove the chassis and the power amp module, etc from the amp. Please be sure to read through it so that you can disassemble the amp safely.

I also found that the main B+ power supply should be somewhere near +90vdc, not the 60-80 that I guessed at.

I don't know what experience you have in working on a powered up amp, but just remember that there are voltages inside the amp that can hurt you. If you work carefully, you should have no problems in making the simple measurements that you need to do on this amp.

There are drawings in the manual that show each of the boards in the amp. The "MA" board is the main power supply. There are solder terminals along the outer edges that connect to different parts of the circuitry. The three terminals that you are looking for are the +12, +36 and B+. The +12 and +36 are next to each other at the upper right hand side of the drawing and should be marked on the board. The B+ is along the left edge and has a wire going to the secondary fuse holder connected to it.

On the board are a number of exposed metal parts that are "live", so be careful with you meter leads and your fingers. There are two fuse holders that carry low ac voltages from the power transformer and there are two large U-shaped metal channels that are the main power supply rectifiers. Treat all of these contact points with care. They may not have the power to do great harm to you, but shorting these things may do great damage to the amp itself.

Good luck!

[KM1]

Thanks Bill for the heads up. I have no experience working with amps, live or otherwise. However, years ago I got nailed by 220 at the box running new wire in. I had one hand wrapped around the metal conduit and the other working the wires into place when the grounds slapped the main bar. That rocked my world and since then have developed a great respect for anything electrical. That said, +36 reads 89.8, while +12 reads 39.6, and +B is nothing. Question: If I blow the fuse immediately won't +B register nothing since the blown fuse is not allowing for charge to pass through? Thank you again for all your help!!!

[J M Fahey]

+36 reads 89.8, while +12 reads 39.6, and +B is nothing.

Sorry but you are measuring wrong.

I bet you are measuring DC voltage on the AC scale, which confuses the meter and makes it show roughly 2.2X what's actually there.

Please recheck.

[52 Bill]

Yes, try the readings again and check to see that your is set to read dc volts.

Let's be certain that we are talking about the same fuse here. The secondary fuse is one of the two fuses on the back panel. It should be marked as secondary. If the secondary fuse is the one that blows, then there should still be voltage on the supply side of the fuse. Do not install a new fuse there, just read the voltage at the "MA" board where the voltage is generated.

[KM1]

Ah, thank you Juan. As I said I am a newbie at this. Okay, so after adjusting my method the readings are thus: +36 = 40.8, +12 = 14.5, +B = 0

[KM1]

Diode tested the output capacitors and one of them is bad. Going to replace that and see if problem is fixed. Thank you guys so much for your input and speedy responses. I wouldn't have gotten this far without you. I will update after. Thanks again!!!

[Enzo]

How do you diode test a cap?

[KM1]

How do you diode test a cap?

It was the output transistor, my mistake.

[Jazz P Bass]

Make sure the 0.47/ 2W ballast resistors are still good.

[KM1]

Make sure the 0.47/ 2W ballast resistors are still good.

I will check those. Thanks!